Draglines are large excavating machines designed to fill, carry and dump loads of material, typically earth. Draglines are often used in open cut coal mines to remove waste overburden covering a shallow coal seam.
FIG. 1 illustrates a typical large electric dragline in accordance with the prior art. A conventional dragline includes a rotatable support 1 mounted on a stationary base 2. An outwardly projecting boom assembly 3 is mounted pivotally to the rotatable support. Winches 6, 9 are mounted on the support for retrieving or releasing cables or ropes. Normally there are two main sets of ropes or cables, hereinafter referred to as hoist ropes 4 and drag ropes 5. Hoist ropes 4 extend from the hoist winch 6 mounted on the support, up and outwardly along the boom, over pulleys or sheaves 7 mounted at the most distant point of the boom, down to a bucket and rigging assembly 8. Drag ropes 5 extend from a drag winch 9 mounted on the support 1, outwardly to the bucket and rigging assembly 8. The bucket and rigging assembly consists of the bucket itself, and the “Rigging” which refers to the total collection of chains, ropes, cables and other components used to suspend the bucket.
A conventional dragline is equipped with a mechanism for locomotion, typically being reciprocating support feet or crawler tracks.
FIG. 2 shows the typical components of the bucket and rigging assembly in accordance with the prior art. While it is acknowledged that there are variations on the arrangements and names of components, the following definitions, familiar to any person skilled in the art, will be used:
Drag ropes 5 which are used to pull the bucket while filling (normally two).
Drag chains 10 which connect the drag ropes to the bucket.
Hoist ropes 4 which are used to lift and carry the bucket (normally two).
Hoist chains 11 (upper and lower) which connect the bucket to the hoist ropes.
Spreader bar 12 which separates the left and right hoist chains to allow the bucket to sit between. It is situated at the junction of the upper and lower hoist chains.
Dump rope 13 that allows the bucket to be picked up or dumped by applying or releasing tension to the drag ropes.
Dump block 14 which is a pulley around which the dump rope is free to move.
Dump chains 15 which are intermediate chains connecting the dump rope to the leading end of the drag chains.
Miracle hitch 16 which is a three way link that connects the hoist ropes, chains and dump block.
Drag three way link 17 that joins the drag ropes, drag chains and dump chains.
Equaliser links 18 that equalise the loads between various components and allow interconnection, e.g., from the two hoist ropes to the single miracle hitch.
Rope sockets 19 that are used to terminate ropes and allow their connection to other components.
Teeth and lip assembly 20 which is the leading (cutting) edge of the bucket.
Basket 21 which is the main body of the bucket used to carry payload.
Arch 22 which provides structural integrity to the bucket and supplies a point to attach the dump rope.
Dump hitch 23 which is the point on the ash to which the dump rope is attached.
Drag hitches 24 which are the points on the front of the bucket to which the drag chains are connected.
Hoist trunnions 25 which are the points to which the lower hoist chains are attached to the bucket.
Top rails 26 which are structural thickeners along the top edges of the bucket.
Rear rail 27 which is a structural thickener along the top edge of the rear of the bucket.
Other relevant definitions are:
“Carry Angle” which is the acute angle between the floor of the bucket and the horizontal.
“Rated Suspended Load” (RSL) which is the maximum recommended load that can be suspended from the hoist ropes.
“Boom Point” which is the most distant extreme of the boom 3 from the support 1. This point corresponds to the location of the Boom Point Sheaves 7.
“Boom Point Radius” which is the horizontal radius measured outwardly from the centre of rotation of the support 1 to a point directly under the boom point sheaves 7.
The drag and hoist ropes may be retrieved or released from their respective winches to move the bucket freely in space. The rotatable support can “Swing” the upper dragline assembly and thus bucket and rigging through a horizontal arc.
The normal operation of a dragline begins with the bucket freely suspended in space above the ground. The bucket is then lowered to the ground and positioned by releasing rope from the hoist winch and/or drag winch. The bucket is then filled with material by retrieving drag ropes onto the drag winch. At some point, the bucket may be lifted or “Disengaged” from the ground by retrieving the hoist ropes. In this operation, tension is developed in the dump rope 13 which causes the front of the bucket to lift via the arch 22. A certain volume of excavated material known as “Payload” is retained in the bucket after disengaging. The bucket may then be moved to its dump point by retrieving and releasing the hoist and drag ropes and/or swinging the support 1. The payload is dumped by releasing drag rope until the dump rope loses tension and allows the bucket to tip forward. This operation can only occur under, or nearly under the boom point sheaves.
For a typical large electric dragline (e.g. BE 1370W or Marion 8050), the bucket capacity is approximately 47 cubic meters. The bucket weight is typically 40 tonnes. The combined rigging weight is typically 20 tonnes. The RSL for these machines is approximately 150 tonnes. Therefore, the manufacturers recommend payloads of approximately 90 tonnes.
There are a number of limitations that conventional rigging designs place on operating a dragline.
a) After filling the bucket, it cannot be disengaged from the ground until the bucket is sufficiently close to the support 1 to allow enough tension to be developed in the dump rope to lift the bucket arch. FIG. 3 shows that if the bucket is lifted too early, the forward section of the payload is lost. This means that the bucket must be “Over-dragged”after it is full, to a point where it can be lifted and retain a satisfactory payload. This adds to cycle time, increases wear and reduces hoisting efficiency.
b) A dragline bucket can only dump at the perimeter defined by the boom point radius. This is because the dump rope will only become slack enough to drop the front of the bucket when the drag rope tension is low, i.e., the drag ropes have been sufficiently released. FIG. 4 shows this effect. There are dynamic methods for dumping just inside and outside of boom point radius, however these methods are not recommended by the manufacturers.
When a bucket is being carried, its carry angle is determined by two main factors: (i) the bucket position with respect to the boom, and (ii) the length of the dump rope. The payload retained in the bucket depends heavily on the carry angle—too shallow and the payload front section is lost,—too steep and the top-rear section is lost. This effect is shown in FIG. 5.
Various proposals have been made to improve the control of the orientation of the dragline bucket in a vertical plane i.e. “carry angle” control by utilising differential control of the two hoist ropes, one of which is operatively connected to the front of the bucket, and the other operatively connected to the rear of the bucket. By adjusting the position of one hoist rope relative to the other, the vertical orientation of the bucket can be adjusted in order to provide a dumping movement without relying on the dump rope becoming slack with all of the disadvantages set out above. Constructions of this type have been proposed in Australian Patent Application 34502/89 (“Beatty”) and in Russian Patent Specifications 972008 and 606945. In both the Beatty and the Russian '008 specifications the carry angle of the bucket is controlled by differential hoist rope movement with the hoist rope entrained over side by side boom point sheaves on a common axis, as is commonly used in dragline construction Beatty, in FIG. 7, shows a construction where the rear hoist rope 63d can be shortened relative to the front hoist rope 63c by using a sheave 58a forced sideways against hoist rope 63d by hydraulic ram 57a to move the bucket from a carrying to a dumping or chopping mode.
Both Beatty and Russian '008 have the disadvantage that they retain a significant number of conventional rigging components such as spreader bars and hoist trunnions, that due to their combined weight, limit the maximum payload that can be carried without exceeding the manufacturers RSL. Furthermore, by positioning the boom point sheaves side by side in the conventional manner, increased loads are placed on the hoist ropes as the bucket is raised to a position approaching the boom due to triangulation between the hoist ropes and the bucket from the spacing apart of the hoist rope attachment points on the bucket. This limits the freedom of movement of the bucket relative to the boom and also causes the bucket carry angle to vary significantly as the drag ropes are retrieved or paid out.
Russian specification 606945 describes an excavator having the bucket suspended by hoist ropes attached to the front and rear of the bucket respectively, and wherein a mechanism is provided at the boom point operable to move the boom point sheave of the rear hoist rope outwardly, shortening the vertical scope of the rear hoist rope relative to that of the front hoist rope to move the bucket from a digging or carrying orientation to a dumping orientation. This configuration has the disadvantage of providing additional complication and significantly increased weight at the boom point, which would significantly reduce the RSL of the excavator. When the bucket is held in the normal carry or dig modes, the sheaves are close together and the problem of increased loads from triangulation is present as for Beatty and Russian '008 (see FIG. 1 of Russian '945). Furthermore, the method proposed in Russian '945 is completely unsuited for use with large electric dragline as the weight of the mechanism at boom point would result on unacceptable loadings on the boom and an unacceptable increase in rotational inertia of the boom and housing assembly when the housing is pivoted in its base for dumping or other similar operations. It is also believed that the mechanism in Russian '945 is totally inapplicable to a large electric dragline as the force required to be developed by the hydraulic ram at boom point would not be available from any known hydraulic ram system.
It has also been proposed at various times to use a computer to control some of the operations of a dragline for various purposes such as the accurate positioning of the dump position over a hopper for the discharging of the bucket load onto a conveyor. Control of this type has been proposed in Australian patent application 87303/77 (“Mitsubishi”) and 28179/84 (Winders, Barlow and Morrison; “WBM”).
Both the Mitsubishi and WBM patent specifications describe the use of a computer to accurately control the transition of the dragline from one mode to another. They are particularly concerned with accurately swinging the dragline from an orientation used for the digging operation to a second orientation used for dumping, and to accurately control the dumping point to ensure that the pay load can be dumped into a hopper strategically placed on a conveyer belt for the removal of material from the area. In this sense, both Mitsubishi and WBM improve the accuracy of the operator by imposing computer controlled parameters at the change over from one mode of operation to the other, but they do not enhance the overall operating efficiency of the dragline by enabling accurate control of the carry angle of the bucket, particularly in the digging, carrying, and cleaning modes.
It is therefore an object of the preset invention to provide dragline bucket rigging and control apparatus which will obviate or minimise some or all of the foregoing disadvantages in a simple yet effective manner or which will at least provide a useful choice.